Along with hundreds of other women and children, she prevented the Lebanese Army and airport employees from reaching the terminal by setting up human barricades backed by rows of boulders and smelly, burning tires that produced clouds of back smoke over the capital. For once, there were no signs of weapons.

Oum Aziz literally wears her cause pinned to her chest. Three photographs safety-pinned to her shirt show the faces of the three sons who have disappeared without a trace.

They are among the more than 3,500 people who were abducted deliberately or because they were in the wrong place at the wrong time since Lebanon's civil war erupted in 1975.

They also represent the cost of nine years of war in human terms, which have surfaced during the country's first full week of peace arranged under the auspices of Syria and the new Lebanese ''National Unity Government.''

The issue of missing persons has become so potent in Lebanon that it has threatened to shatter the unfamiliar calm in the capital.

The problem is not just that thousands - some unofficial estimates go as high as 7,000 - are missing. The vast majority have probably been killed in detention , according to the militias.

''First indications are that Lebanon is going to live with a very tragic problem of missing people - like Argentina and Chile,'' explained Marwan Hamadeh , a former Cabinet minister and now a leading Druze faction official, in an interview with a Beirut magazine.

''Fears are growing that the numbers of people who might be released will be only a very small proportion of those kidnapped. I think everybody is afraid to face this reality because everybody is afraid to face the families of those who have been kidnapped.''

Mr. Hamadeh called the situation ''a permanent shame on all parties to Lebanon's civil war.''

Serge Cassia of the International Red Cross confirmed that officials involved in organizing the release had seen fewer than 200 hostages held by the three main Christian and Muslim militias.

The discrepancy in numbers and the delay in release triggered four days of demonstrations by the informal ''Relatives of the Missing'' organization, which included blocking off the recently reopened crossing between the Christian east and Muslim west halves of the capital.

The Christian Lebanese Forces militia claim 1,460 Christians have been kidnapped, while the Shiite Muslim Amal movement and the Druze Progressive Socialist Party are demanding the return of 2,100. And those names represent only the past two years.

Most of those not released during the last short-lived spurt of calm that was orchestrated by American mediators in the fall of 1982 are believed to be dead.

Druze leader Walid Jumblatt, who is a member of the 10-week-old Cabinet, admitted Wednesday: ''We have to be frank today with the citizens and solve the issue of the hostages. We've received a list of 1,400 Christians (missing). We do not have live hostages.''

The missing have been traced to a variety of places and methods of abduction. Hundreds of those on the Christian list disappeared during the Shouf ''war of the mountains'' last fall and the battle for west Beirut in February. Both conflicts pitted Christian gunmen against Muslim forces, and in both the Druze and Shiite militias won.

Most of the Muslim missing date back further, to late 1982 and early 1983, when Christians had the upper hand, both militias and the then-Christian-dominated Army.

On both sides of the ''green line'' that divides Beirut, some of the victims were taken by happenstance, caught at any of the many checkpoints run by Christian or Muslim militias on both main thoroughfares and side streets.

Lebanese government-issued identity cards make it impossible to hide religion , which is clearly marked on the fifth line of the blue passes each citizen must carry at all times. Eliminating confessional identity is one of the main demands of Muslim groups seeking constitutional reforms.

The latest case involved a female student at the American University of Beirut who did not come home from classes last week. Like many families without any alternative means of recourse, her mother placed an advertisement in a local paper offering a reward for information about what happened.

Indeed, one of the biggest rackets in a nation riddled with corruption involves con men who pledge, for fees up to $4,000, either to provide information about or win early freedom for a missing person. Interviews with families who have paid out significant sums of money indicate they never saw the men again - or the missing relatives.

The minority of victims, according to diplomats and relief agencies, were actually deliberately abducted because of positions within opposing militias. Most appear to have been civilians.

None of the lists now being assembled include the Palestinians, more than 350 of whom have been unaccounted for since the autumn 1982 massacre in the refugee camps of Sabra and Shatila. It was allegedly Christian gunmen who entered the camps.

Under pressure from the government and the various militias, Oum Aziz and her friends on Tuesday finally moved their demonstrations from Beirut's strategic junctions to religious centers. But the issue is not over.

On Wednesday the Cabinet of Prime Minister Rashid Karami announced it would set up a committee to investigate each case of missing people. ''The program calls for all to be released,'' Mr. Karami said.

The danger for Lebanon is that after the release, which could be as early as next week, families of those who are not returned will begin reprisals.

A former Lebanese prime minister has predicted often during the past two years, ''Everyone in Lebanon now wants justice. The problem is that for the Lebanese justice now means revenge.''